Pyrotechnic gas-generating composition intended for motor vehicle safety and burning at combustion temperatures below 2200 degree K

ABSTRACT

The present invention relates to a gas-generating pyrotechnic composition, this composition being characterized in that it comprises a first oxidizing charge consisting of ammonium perchlorate and a second oxidizing charge consisting of a metal oxide chosen from the group consisting of copper oxide, manganese oxide and a mixture of these two compounds, the weight ratio between the first oxidizing charge and the second oxidizing charge being less than 1, the said composition also including a reducing charge in the form of a nitrogen-containing organic compound chosen from the group consisting of nitroguanidine, guanidinium nitrate, oxamide, dicyandiamide and metal cyanamides.

The present invention relates to the pyrotechnic generation of gases, in particular for inflating airbags used in systems for protecting the occupants of a motor vehicle. The invention relates more particularly to pyrotechnic compositions generating clean, so-called “cold”, gases that are rich in nitrogen and are non-toxic, at temperatures acceptable for motor vehicle safety.

For different pyrotechnic requirements and in particular to ensure correct inflation of airbags, pyrotechnic gas generators must provide, in extremely short periods of time, of the order of thirty milliseconds, clean gases, namely those free from solid particles likely to constitute hot spots that can damage the wall of the airbag, and that are non-toxic, namely having low contents of nitrogen oxides, carbon oxides and chlorinated products.

Various families of compositions have been developed for this purpose. One of these families concerns “composite” compositions consisting fundamentally of an organic binder and an oxidizing charge such as in particular ammonium perchlorate. These compositions are very valuable since they have a good combustion rate and excellent stability on ageing.

Compositions have thus been proposed in patent FR 2 137 619 or in its corresponding patent U.S. Pat. No. 3,723,205 of which the binder is a polyvinyl chloride and of which the oxidizing charge is an ammonium perchlorate in the presence of sodium nitrate as an internal chlorine scavenger. Nevertheless, the use of a chlorinated binder in the presence of high energy charges is difficult to implement, in particular as regards safety and non-toxicity of the gases generated.

Compositions have also been proposed based on ammonium perchlorate and sodium nitrate mixed with nitrogen-containing compounds such as azides or metal nitrides. Compositions of this type are for example described in patent U.S. Pat. No. 3,814,694. However, on account of the presence of azides that make them very toxic in operation, these compositions are not recommended.

Patent U.S. Pat. No. 4,909,549 describes compositions consisting of a mixture of ammonium perchlorate and sodium nitrate associated with a nitrogen-containing compound of triazole or of tetrazole. Such compositions indeed generate clean gases that are rich in nitrogen but these gases are relatively toxic and must be diluted with air in order to be used in motor vehicle safety.

Compositions have also been proposed consisting of a silicone binder and a mixture of ammonium perchlorate and sodium nitrate. Such compositions, described for example in patent FR 2 728 562 or in its corresponding American patent U.S. Pat. No. 5,610,444, indeed generate clean gases that are rich in nitrogen and are non-toxic but they have the disadvantage of burning at very high temperatures.

A gas-generating pyrotechnic composition is also known from patent application U.S. Pat. No. 6,190,474, burning at relatively low temperatures of between 2000 and 2200 K, comprising in particular a reducing charge consisting of nitroguanidine and an oxidizing agent composed for example of an alkali metal nitrate or alkaline-earth metal nitrate or a metal oxide or a mixture of the two. This composition comprises for example 20 to 80% by mass of nitroguanidine and 80 to 20% by mass of oxidizing agent. This type of composition generates gases at temperatures below 2200 K, which constitutes an advantage for use in motor vehicle safety. However, the combustion rates of these compositions remain too low for them to be able to be used effectively in gas generators intended for motor vehicle safety.

Patent FR 2 772 370 or its corresponding American patent U.S. Pat. No. 6,533,878 describes a composition generating clean and non-toxic gases at acceptable temperatures for motor vehicle safety, namely below 2200 K. This composition more particularly includes a crosslinked reducing binder and a principal oxidizing charge consisting of a mixture of ammonium perchlorate associated with a chlorine scavenger chosen from the group consisting of sodium nitrate, potassium carbonate and lithium carbonate. This composition also includes additives containing at least one copper compound such as copper oxide and a nitrogen-containing organic compound such as for example nitroguanidine or guanidinium nitrate. This composition burns at combustion rates and at temperatures completely suited to use in motor vehicle safety.

This patent application describes certain compositions in which ammonium perchlorate is the main constituent. Now, even though it is true that ammonium perchlorate is very oxidizing and has a very good gas yield, it increases the combustion temperature of the composition and contributes considerably to an increase in the creation of chlorine-based products such as hydrogen chloride, these products forming droplets and particles that are difficult to filter and are the source of copious fumes. Now, the formation of fumes is a phenomenon that should be limited to the maximum.

A gas-generating composition is also known from patent application DE 298 21 541 (corresponding to EP 1 006 096) comprising 20 to 60% of a fuel and 40 to 80% of an oxidizing mixture. The fuel includes at least 95% of a nitrogen-containing organic compound (guanidine nitrate), while the oxidizing mixture comprises 20 to 80% of a metal oxide, 0 to 50% of basic copper nitrate, 1 to 15% of a metal chlorate or ammonium perchlorate and 1 to 15% of an alkali metal nitrate or an alkaline-earth metal nitrate. This composition has a low ammonium perchlorate content which makes it possible to limit the formation of fumes and the production of hydrogen chloride (HCl). However, ammonium perchlorate makes it possible in particular to confer on the composition a good gas yield, a higher combustion rate and good ignitability. Moreover, by regulating the particle size of the ammonium perchlorate, it is possible to regulate the ballistics of the composition. The use of a small ammonium perchlorate content in the composition described in application DE 298 21 541 does not make it possible to profit fully from these advantages. Finally, this composition that is based on the major use of metal oxides can only be employed by pelleting. The low content of ammonium perchlorate, which has a high oxygen balance, does not allow a binder to be used in a very large quantity and therefore does not enable extrusion to be employed.

In application EP 1 130 008 (corresponding to U.S. Pat. No. 6,592,778) a gas-generating composition is described comprising 45% guanidine nitrate, 13% copper oxide, 25% basic copper nitrate, 4% ammonium perchlorate, 3% sodium nitrate, 10% iron oxide and 24% of a 2% solution of hydroxyethylcellulose. Hydroxyethylcellulose is used as a thickening agent for the composition. As in application DE 298 21 541, the composition described in this application EP 1 130 008 has a low ammonium perchlorate content. This composition therefore has the. same disadvantages as that previously described. However, this document describes a method permitting production by extrusion by using a thickening agent.

The object of the invention is therefore to provide a pyrotechnic composition generating clean, non-toxic gases at low temperatures lower than 2200 K, of value in ballistics, guaranteeing a good gas yield and good ignitability and creating very little fumes and very little hydrogen chloride.

This object is achieved with a gas-generating pyrotechnic composition, this composition being characterized in that it comprises a first oxidizing charge consisting of ammonium perchlorate present at a weight concentration between 15 and 30% of the total weight of the composition, and a second oxidizing charge consisting of a metal oxide chosen from the group consisting of copper oxide, manganese oxide and a mixture of these two compounds, the weight ratio between the first oxidizing charge and the second oxidizing charge being less than 1, the said composition also including a reducing charge in the form of a nitrogen-containing organic compound chosen from the group consisting of nitroguanidine, guanidinium nitrate, oxamide, dicyandiamide and metal cyanamides.

According to the invention, the concentration by weight of the first oxidizing charge, namely ammonium perchlorate, lies between 15 and 30% of the total weight of the composition. Ammonium perchlorate is a very strong oxidant and has a very good gas yield. Its strongly oxidizing character makes it possible in particular to increase the proportion of reducing charge and therefore for example of guanidinium nitrate in the composition, the latter also having itself a very good gas yield. The presence of ammonium perchlorate in the composition enables a high combustion rate to be obtained for the composition. The combustion rate can be regulated by the particle size of the ammonium perchlorate present in the pyrotechnic compound to be burnt. The combustion rate can also be regulated by the use of catalysts for the combustion of the composition. The fact of having available a composition of which the rate of combustion is by the nature of its constituents very high, presents a certain advantage. Indeed, in motor vehicle safety, virtually all the pyrotechnic composition has to have been burnt, under a pressure of the order of 200 MPa, in 30 to 40 milliseconds for use in a passenger or driver airbag or in 10 milliseconds for use in a side airbag. These combustion times constitute an essential requirement to be met in compositions intended for motor vehicle safety. Since the combustion time is directly the quotient of the thickness to be burnt over the rate of combustion, it will immediately be seen that if the combustion rate of the composition is high, it will be possible to produce thicker and therefore possibly extrudable monolithic blocks, or thicker pellets that are therefore simpler to manufacture, while preserving short combustion times. According to the invention, the use of ammonium perchlorate, that gives the composition a high combustion rate, also makes it possible to be independent of thickness variations that can occur on the compound to be burnt during its manufacture and therefore makes it possible to avoid discrepancies in operation associated with these variations during the combustion of the compound.

Moreover, when the pyrotechnic product obtained burns in parallel layers, the combustion rate V_(c) satisfies the following law: V_(c)=a×p^(n) in which a is a constant and n the pressure exponent. The combustion of a pyrotechnic compound is above all a chemical reaction and is therefore dependent on temperature. Now the use of ammonium perchlorate makes it possible to reduce the pressure exponent, which makes the composition less dependent on pressure and therefore less dependent on temperature. According to the invention, the pressure exponent is particularly low, largely less than 0.7, so that the pyrotechnic compound can be employed at temperatures between −35° C. and 85° C. It will then be possible to use the composition according to the invention in a motor vehicle.

The composition according to the invention does not have to include a binder. However, in order to confer good mechanical strength on the final pyrotechnic compounds obtained, such as pellets, discs or monolithic blocks, a binder can be added to the composition.

It is known that in the field of motor vehicle safety, the compositions must have an oxygen balance (called OB) that is equilibrated. By definition, a composition is said to be balanced in oxygen when the composition has sufficient oxygen so that, after reaction, the various compounds of the composition are found in the form of CO₂, H₂O and N₂. The oxygen balances of the binders are generally very low and often have to be compensated for. The presence of ammonium perchlorate in a composition makes such a compensation possible. However, ammonium perchlorate should not be at a very high concentration in the composition so as to prevent the disadvantages associated with its use such as the production of hydrogen chloride or the creation of fumes.

The oxygen balance of the binder should be as low as possible. An adjustment of the oxygen balance around equilibrium also makes it possible to limit the quantity of chlorides formed.

According to the invention, the relatively high proportion of ammonium perchlorate in the composition according to the invention allows a binder to be incorporated at a sufficiently high level for the composition to be employed by extrusion.

According to one embodiment of the invention, the composition includes a binder chosen from the group consisting of a methylcellulose and polyvinyl alcohol. According to a preferred embodiment of the invention, the selected binder is sodium carboxymethylcellulose (from now on Na-CMC). In point of fact, according to the invention, this binder has in particular good strength on ageing measured according to the ageing test carried out for 400 hours at 107° C.

According to the invention, the selected binder is sodium carboxymethylcellulose. This binder has an oxygen balance of −103.5% (0.7 degree of substitution) which is in particular very much higher than the oxygen balance of binders of the elastomeric type. Sodium carboxymethylcellulose binder also lends itself very well to extrusion, to granulation operations and to compression and will therefore be perfectly suited to being employed in the composition in the form of pellets, discs or monolithic blocks.

The composition described in the prior patent application FR 2 772 370, includes a crosslinked reducing binder based on silicone resin or based on epoxy resin. The binder is present in this composition at a concentration of between 6% and 10% of the total weight of the composition. So that it can be obtained by extrusion with a twin-screw extruder, a composition should include a minimum binder content, of the order of 4 or 5% of the total weight of the composition, and requires the use of a solvent for the binder. The selected binders in this composition of the prior art make it necessary to use organic or halogenated solvents. Now, the use of halogenated solvents such as for example trichloroethylene, is subject to regulations, which considerably complicate the manufacture of such a composition and increase its manufacturing cost. Similarly, the use of organic solvents leads to the establishment of complex solutions for controlling emissions of volatile organic compounds (VOC).

According to the invention, the binder, such as for example sodium carboxymethylcellulose, is water-soluble, which makes it possible to avoid having to resort, as in the prior art, to halogenated or organic solvents during granulation or extrusion steps participating in the manufacture of pyrotechnic compounds.

According to one special feature, the weight concentration of the binder represents a maximum of 6% of the total weight of the composition. As described above, the binder has, as a general rule, a very low oxygen balance. So as not to penalize the oxygen balance of the composition too much, the proportion of binder in the composition must remain very small.

According to another special feature, the composition includes additives facilitating its manufacture. These additives are for example calcium stearate and make it possible to facilitate the manufacture of the composition according to the invention when it contains no binder.

According to one embodiment of the invention, the nitrogen-containing organic compound, chosen from nitroguanidine, guanidinium nitrate, oxamide, dicyandiamide and metal cyanamides, has a weight concentration between 30 and 50% of the total weight of the composition. The nitrogen-containing organic compound in the composition is the reducing charge and is an integral part of the composition, the binder, when present, then taking part more as an additive. The selected reducing charge is preferably guanidinium nitrate. In point of fact, guanidinium nitrate exhibits very good resistance to ageing measured by the 400 hours test at 107° C. Moreover, the presence of guanidinium nitrate in the composition improves the gas yield of the composition. Guanidinium nitrate has a negative enthalpy of formation, also having the effect of lowering the combustion temperature of the composition.

According to a preferred embodiment of the invention, the weight concentration of the second oxidizing charge lies between 30 and 45% of the total weight of the composition. The presence of this second oxidizing charge makes it possible in particular to limit the quantity of ammonium perchlorate in the composition. Even though, as described above, ammonium perchlorate is a strong oxidant and exhibits valuable properties in combustion, its use in a composition leads in point of fact to high combustion temperatures being obtained and the generation of combustion gas having quite a high hydrogen chloride content. This second oxidizing charge is a metal oxide and preferably copper oxide CuO or manganese oxide MnO₂. A mixture of these two oxides can also be used as the second oxidizing charge. This second oxidizing charge is present at a very high level in the composition and at a higher level than that of ammonium perchlorate. It has a low enthalpy of formation so that the combustion temperature of the composition is lowered. In point of fact, for example, the enthalpy of formation of copper oxide CuO is −155 kJ/mol. Copper oxide also has the advantage of being an excellent combustion catalyst for ammonium perchlorate which enables the composition to be given a high combustion rate, this making it very suitable for use in motor vehicle safety.

According to the invention, the composition can also include agents for trapping solid particles emitted during combustion so as to create residues with a sufficiently large size for them to be filtered. Well-known additives in the field of compositions for motor vehicle safety, such as for example alumina or silica can be added to the composition according to the invention.

According to one special feature, the metal cyanamides that may be present in the composition consist of the cyanamides of sodium, zinc and copper. The chosen metal cyanamide is preferably zinc cyanamide ZnCN₂.

According to the invention, pyrotechnic compounds having a composition as described above can be manufactured at a high rate by disc compression or by pelleting. A powder preparative step should be carried out before the pelleting operation. This step does not simply amount to dry mixing the different constituents. Indeed, in order to be able to carry out the pelleting operation, it is necessary to obtain a powder that flows well. This preparative step is a granulating operation consisting of starting with the different materials entering into the make-up of the composition according to the invention and present in powder form, in order to make grains with a larger particle size, of the order of a few hundreds of microns. The pelleting operation can be carried out once this powder has been obtained.

According to the invention, the pyrotechnic products can also be obtained by extrusion with the aid of a twin-screw extruder. For this, it is necessary to incorporate a binder, such as sodium carboxymethylcellulose, into the composition and to use a solvent. As described above, contrary to some binders and in particular those used in the prior art, sodium carboxymethylcellulose is water-soluble. The use of sodium carboxymethylcellulose does not therefore necessitate the use of halogenated or organic solvents. A small proportion of water is incorporated in the mixture so as to prevent solubilization of ammonium perchlorate. According to the invention, the method for preparing pyrotechnic compounds having a composition according to the invention as described above, first of all includes a step of continuously feeding a blending and extruding apparatus, such as for example a twin-screw extruder, on the one hand with the two oxidizing charges and, on the other hand, with the nitrogen-containing organic compound premixed with the binder, namely with sodium carboxymethylcellulose. After blending with the aid of the extruder, a profiled nod is extruded that is oven-cured and cut to the desired length to form aggregates, for example.

The table given below shows several examples of the formulation for the composition according to the invention and the performance indicated for each of these formulations. Na- G O.B. CMC binder Glyc. A.P. MnO₂ CuO nitrate Temp (K) (%) Yield Res. 0 23.8 30 46.2 2230 −1 28.5 12.4 1 25.1 30 43.9 2243 −1 28.2 12.2 1 16.6 40 42.4 2064 −2.5 25.4 24.7 1 20 33 46 2180 −3 28.2 23.1 2 26.4 30 41.6 2257 −1 28 12 2 23.4 35 39.6 2180 −1 26.2 18.2 2 23.1 30 44.9 2230 −3 29 22.3 4 24.3 35 36.7 2200 −2 26.1 17.8 4.5 0.5 25.7 35 34.3 2215 −2 25.8 17.4 1.45 0.15 16.6 40 41.8 2069 −3 25.5 24.4 2 26.4 30 41.6 2062 −1.2 28.3 26.3 2 26.4 15 15 41.6 2190 −1 27.9 13.1 2 26.4 20 10 41.6 2156 −1.1 28.1 17.5

The following abbreviations have been employed in this table:

-   -   Na-CMC=sodium carboxymethylcellulose (quantity expressed in         percent)     -   Glyc.=glycerine (quantity expressed in percent)     -   A.P.=ammonium perchlorate (quantity expressed in percent)     -   MnO₂=manganese dioxide (quantity expressed in percent)     -   CuO=copper oxide (quantity expressed in percent)     -   G. nitrate=guanidinium nitrate (quantity expressed in percent)     -   T=temperature (measured in K)     -   O.B.=oxygen balance (indicated in percent)     -   Yield=gas yield (indicated in moles per kg of composition burnt)     -   Res.=overall concentration of solid residues in the gases at         1000 K and 1 bar, expressed in percent (%) (the temperature of         1000 K corresponds substantially to the temperature of the gases         leaving a gas generator).

It should be noted that a certain quantity of glycerine can be added to the composition.

The following results were obtained when 24 grams of a composition comprising 2% Na-CMC binder, 26.4% ammonium perchlorate, 30% copper oxide and 41.6% guanidinium nitrate, for a composition mass of 24 grams, were fired into a 60 litre tank. −35° C. +20° C. +85° C. P_(tank max) 0.215 0.228 0.251 (MPa) CO (ppm) 1850 2040 1960 NOx (ppm) 660 670 740

The following table gives the results obtained when 18 grams of a composition comprising 2% Na-CMC binder, 26.4% ammonium perchlorate, 30% copper oxide and 41.6% guanidinium nitrate at t₀, were fired into a 60 litre tank after ageing for 400 hours at 107° C. and after cycling. Cycling consisted here of applying 183 cycles to the composition defined above, one cycle being composed of:

-   -   holding the composition for 40 minutes at −40° C.,     -   raising the temperature in 60 minutes to 107° C.,     -   holding this temperature of 107° C. for 40 minutes,

lowering the temperature in 60 minutes to −40° C. After 400 h After t₀ at 107° C. cycling Vc (20 MPa) in 29.6 31 30 mm/s n (15-30 MPa) 0.30 0.42 0.41 Firings into 60 liter tank (18 g of composition): CO (ppm) 2300 2320 2730 NOx (ppm) 520 420 430 P_(max tank) (MPa) 0.170 0.183 0.176 n = pressure exponent

After ageing for 400 hours at 107° C., the composition has lost 0.24%. of its overall mass. The composition had already lost 0.23% of its overall mass at the end of 200 hours. After this ageing for 400 hours at 107° C., it was also noted that the composition had an equivalent performance to that existing before ageing.

It should be obvious to persons skilled in the art that the present invention makes possible embodiments under many other specific forms without departing from the application field of the invention as claimed. Consequently, the present embodiments should be considered as an illustration, but can be modified within the field defined by the scope of the accompanying claims, and the invention should not be limited to the details given above. 

1. Gas-generating pyrotechnic composition, characterized in that it comprises a first oxidizing charge consisting of ammonium perchlorate present at a weight concentration between 15 and 30% of the total weight of the composition, and a second oxidizing charge consisting of a metal oxide chosen from the group consisting of copper oxide, manganese oxide and a mixture of these two compounds, the weight ratio between the first oxidizing charge and the second oxidizing charge being less than 1, the said composition also including a reducing charge in the form of a nitrogen-containing organic compound chosen from the group consisting of nitroguanidine, guanidinium nitrate, oxamide, dicyandiamide and metal cyanamides.
 2. Composition according to claim 1, characterized in that it includes a binder chosen from the group consisting of a methyl cellulose and polyvinyl alcohol.
 3. Composition according to claim 2, characterized in that the binder is sodium carboxymethylcellulose (Na-25 CMC).
 4. Composition according to claim 2, characterized in that the weight concentration of the binder represents a maximum of 6% of the total weight of the composition.
 5. Composition according to claim 1, characterized in that the composition includes additives facilitating its manufacture.
 6. Composition according to claim 1, characterized in that the nitrogen-containing organic compound has a weight concentration between 30 and 50% of the total weight of the composition.
 7. Composition according to claim 1, characterized in that the weight concentration of the second oxidizing charge lies between 30 and 45% of the total weight of the composition.
 8. Composition according to claim 1, characterized in that the metal cyanamides consist of the cyanamides of sodium, zinc and copper.
 9. Composition according to claim 8, characterized in that the metal cyanamide is zinc cyanamide ZnCN₂.
 10. Composition according to claim 1, characterized in that it includes additives acting as combustion catalysts or agents for trapping solid particles emitted during combustion.
 11. Composition according to claim 1, characterized in that it is manufactured and formed by a pelleting or disc compression process.
 12. Composition according to claim 2, characterized in that it is manufactured and formed by an extrusion process.
 13. Composition according to claim 3, characterized in that the weight concentration of the binder represents a maximum of 6% of the total weight of the composition. 